1. Field of the Invention
The present invention relates to a luminance signal and color signal separating filter and, more particularly, to a motion adaptive luminance signal and color signal separating filter for separating a luminance signal (hereinunder referred to as "Y signal" or "Y") and a color signal (hereinunder referred to as "C signal" or "C" from a composite color television signal (hereinunder referred to as "V signal") in which the frequency of a C signal is multiplexed in a high-frequency region of a Y signal.
The motion adaptive YC separating filter is a filter which executes YC separation suitable to the pixel signal at each part while locally judging whether the picture is a still picture or a motion picture.
2. Description of the Related Art
In the current NTSC system, the frequency of a C signal is multiplexed in a high-frequency region of a Y signal in a V signal. YC separation is therefore necessary in a television set. Further, if the separation is insufficient, a deterioration in the picture quality such as cross color and dot crawl is caused.
Various signal processing circuits for improving the picture quality have hitherto been proposed. One example of them is a motion adaptive YC separating filter which utilizes a delay circuit having a delay time equal to or greater than the vertical scanning frequency of a television signal.
FIG. 12 is a block diagram showing an example of the structure of a conventional motion adaptive YC separating filter. In FIG. 12, a V signal 101 of an NTSC system is input from an input terminal 1, and a Y signal and a C signal are output from output terminals 2 and 3, respectively. The V signal 101 is also supplied to the input terminals of an infield YC separating circuit 4, interframe YC separating circuit 5, Y signal movement detecting circuit 6 and a C signal movement detecting circuit 7.
An infield separated Y signal (hereinunder referred to as "Yf signal") 102 and an infield separated C signal (hereinunder referred to as "Cf signal") 103 which are separated from each other by an infield filter (not shown) of the infield YC separating circuit 4 are input to a first input terminal of a Y signal mixer 9 and a first input terminal of a C signal mixer 10, respectively.
An interframe separated Y signal (hereinunder referred to as "YF signal") 104 and an interframe separated C signal (hereinunder referred to as "CF signal") 105 which are separated from each other by an interframe filter (not shown) of the interframe YC separating circuit 5 are input to a second input terminal of the Y signal mixer 9 and a second input terminal of the C signal mixer 10, respectively.
A synthesizer 8 synthesizes a signal 106 of amount of Y signal movement which is detected by the Y signal movement detecting circuit 6 and input to one input terminal of the synthesizer 8 and a signal 107 of amount of C signal movement which is detected by the C signal movement detecting circuit 7 and input to the other input terminal of the synthesizer 8. A movement detecting signal 108 obtained by the synthesis of the signals 106 and 107 by the synthesizer 8 is input to a third input terminal of the Y signal mixer 9 and a third input terminal of the C signal mixer 10.
The Y signal movement detecting circuit 6, the C signal movement detecting circuit 7 and the synthesizer 8 constitute a movement detecting circuit 92.
A motion adaptive separated Y signal 109 which is output from the Y signal mixer 9 is supplied from the output terminal 2. Further, a motion adaptive separated C signal 110 which is output from the C signal mixer 10 is supplied from the output terminal 3.
The operation of the YC separating circuit having the above-described structure will now be explained.
The movement detecting circuit 92 synthesizes the outputs of the Y signal movement detecting circuit 6 and the C signal movement detecting circuit 7 by the synthesizer 8 and judges whether the V signal 101 is a signal indicating a still picture, or a signal indicating a motion picture before separating the Y signal and the C signal from the input V signal 101.
FIG. 15 is a block diagram showing an example of the structure of the Y signal movement detecting circuit 8. A difference between the V signal 101 which is input from a terminal 73 and a V signal delayed by 1 frame is obtained by a 1-frame delay circuit 75 and a subtracter 76. Then, after the difference signal is passed through a low pass filter (hereinunder referred to as "LPF") 77, the absolute value of the difference signal is obtained by an absolute value circuit 78. The absolute value is converted into the signal 106 showing the amount of movement of the low frequency component of a Y signal by a nonlinear converter 79. The converted signal is then to a terminal 74.
FIG. 13 is a block diagram showing an example of the structure of the C signal movement detecting circuit 7. A difference between the V signal 101 which is input from the terminal 61 and a V signal delayed by 2 frames is obtained by a 2-frame delay circuit 64 and a subtracter 65, and after the difference signal is passed through a band pass filter (hereinunder referred to as "BPF") 66, the absolute value of the difference signal is obtained by an absolute value circuit 67. The absolute value is converted into the signal 107 showing the amount of movement of a C signal by a nonlinear converter 68 and the converted signal is output to a terminal 62.
The synthesizer 8 selects the larger value from the signal 106 of amount of Y signal movement and the signal 107 of amount of C signal movement and outputs the selected signal.
The result of judgement is represented in the form of a movement coefficient (0.ltoreq.k.ltoreq.1) and is supplied to the Y signal mixer 9 and the C signal mixer 10 as a control signal 108. For example, when the picture is judged to be a still picture, k=0, and when the picture is judged to be a complete motion picture, k=1.
Generally, when the picture is a still picture, the Y signal is separated from the C signal by the interframe YC separation utilizing interframe correlation.
FIG. 16 is a block diagram showing an example of the detailed structure of the interframe YC separating circuit 5. A sum of the V signal 101 which is input from a terminal 80 and a V signal delayed by 1 frame is obtained by a 1-frame delay circuit 83 and an adder 84. The YF signal 104 is extracted and output to a terminal 81 and simultaneously the CF signal 105 extracted by subtracting the YF signal 104 from the input V signal 101 is output to a terminal 82.
Generally, when the picture is a motion picture, the Y signal is separated from the C signal by the infield YC separation utilizing infield correlation.
FIG. 17 is a block diagram showing an example of the detailed structure of the infield YC separating circuit 4. A sum of the V signal 101 which is input from a terminal 86 and a V signal delayed by 1 line is obtained by a 1-line delay circuit 89 and an adder 90. The Yf signal 102 is extracted and output to a terminal 87 and simultaneously the Cf signal 103 extracted by subtracting the Yf signal 102 from the input V signal 101 is output to a terminal 88.
As described above, the motion adaptive YC separating filter is provided with the infield YC separating circuit 4 and the interframe YC separating circuit 5 disposed in parallel to each other, and outputs the motion adaptive separated Y signal 109 obtained from the Y signal mixer 9 which executes the following calculation on the basis of the movement coefficient k synthesized by the synthesizer 8: EQU Y=kYf+(1-k)YF
wherein
Yf: the infield separated Y signal output 102 PA1 YF: the interframe separated Y signal output 104. PA1 Cf: the infield separated C signal output 103 PA1 CF: the interframe separated C signal output 105.
In the same way, the motion adaptive separated C signal 110 obtained from the C signal mixer 10 which executes the following calculation on the basis of the control signal 108 is output: EQU C=kCf+(1-k)CF
wherein
The motion adaptive separated Y signal 109 and the motion adaptive separated C signal 110 are supplied from the output terminal 2 and 3, respectively.
The C signal movement detecting circuit 7 of such a motion adaptive YC separating filter may also have the structure shown in FIG. 14.
In FIG. 14, the V signal 101 input from the input terminal 61 is demodulated into two kinds of color difference signals R-Y and B-Y by a color demodulator 69. These color difference signals R-Y and B-Y are time-division multiplexed at a certain frequency by a time division multiplexer 70 and a difference between the V signal 101 and a V signal delayed by 2 frames is obtained by the 2-frame delay circuit 64 and the subtracter 65. The thus-obtained difference is passed through an LPF 71 so as to remove the Y signal component, and the absolute value is obtained by the absolute value circuit 67. The thus-obtained absolute value is nolinearly converted by the nonlinear converter 68 and the signal 107 of amount of C signal movement is supplied from the output terminal 62.
The conventional motion adaptive YC separating filter having the above-described structure mixes the Yf signal and the Cf signal obtained by the infield YC separation and the YF signal and the CF signal obtained by the interframe YC separation, respectively, on the basis of the amount of movement detected by the Y signal movement detecting circuit and synthesized with the amount of movement detected by the C signal movement detecting circuit. As a result, the filter characteristic in a still picture and the filter characteristic in a motion picture are completely different from each other. Thus, when a picture is shifted from a still picture to a motion picture or from a motion picture to a still picture, there is an extreme change in resolution. This leads to a remarkable deterioration in the picture quality.